The present invention relates to a method and a system for controlling a behavior of a vehicle, in particular to a method and a system for controlling the yaw moment of a vehicle so as to achieve a desired responsiveness and a high stability of a vehicle.
Four-wheel steering systems (which are abbreviated as 4WS hereinafter) which steer both front and rear wheels have already been put into practical use as a technology for improving the response and stability of a traveling vehicle. Also put into practical use are anti-lock brake systems (which are abbreviated as ABS hereinafter) which control the braking force so as not to lock up the tires by detecting a slip ratio between the wheels and the road surface, and traction control systems (which are abbreviated as TCS hereinafter) which control the traction so as not to freely spin the wheels on the road surface having a low frictional coefficient.
However, although a 4WS is highly effective in improving the responsiveness and stability of a vehicle in a linear region where a linear relationship holds between the slip angle and the cornering force, and the adhesive force between the tires and the road surface is substantially smaller than the maximum possible grip force of the tires, upon onset of a non-linear relationship between the slip angle and the cornering force (when the vehicle is traveling under extreme conditions where the grip force of the tires almost reaching its limit, or when the vehicle is traveling over a low-xcexc road surface), because even though all of the four wheels are steered, the slip angles of the wheels become unable to make any further contribution in increasing the cornering force.
Also, when a braking force or a traction is applied to the wheels, this alone can reduce the cornering force, and thereby affect the turning behavior of the vehicle. Conventionally, the control actions for 4WS, ABS and TCS were individually carried out, and these control systems were therefore unable to comprehensively and/or jointly improve the response and stability of the vehicle under all conditions.
In view of such problems of the prior art, a primary object of the present invention is to provide a method and a system which can maximize the responsiveness and stability of a vehicle for each given traveling condition of the vehicle.
A second object of the present invention is to provide a method and a system which can maintain the favorable responsiveness and the stability of a vehicle even when the traveling condition of the vehicle is such that the dynamic properties of the tires are in a non-linear region.
A third object of the present invention is to provide a method and a system which can achieve a maximum improvement of the handling of a vehicle with a minimum expense.
According to the present invention, these and other objects of the present invention can be accomplished by providing a method for controlling a vehicle behavior by individually controlling a fore-and-aft force of each of right and left vehicle wheels so as to achieve a desired vehicle behavior, comprising the steps of: computing a yawing moment which a vehicle is desired to produce according to a dynamic state quantity of the vehicle including at least a traveling speed, and cornering forces which the wheels produce; and controlling a fore-and-aft force which is applied to each of the wheels so as to achieve said computed yawing moment.
By thus appropriately distributing a fore-and-aft force such as a braking force or a traction to each of the wheels, it is possible to control the vehicle in such a manner which has heretofore not been possible. The cornering forces may be directly detected by using suitable sensors, but may also be estimated from a dynamic state quantity of the vehicle such as the frictional coefficient of the road surface. Therefore, it is made possible to favorably control the vehicle in a more positive manner, and even under most extreme conditions. For the modes of controlling the fore-and-aft force of each of the wheels, reference should be made to U.S. Pat. No. 5,417,298 issued May 23, 1995 to Y. Shibahata, and U.S. Pat. No. 5,474,369 issued Dec. 12, 1995 to H. Inagaki et al, both of which are assigned to the common Assignee. The contents of these patents are hereby incorporated in this application by reference.
Typically, a desired yaw response is determined in according to a dynamic state quantity of the vehicle, and the vehicle is controlled in such a manner that the desired yaw response may be achieved. It is also possible to control the fore-and-aft forces of the wheels so as to achieve a desired vehicle body slip angle.
It was confirmed that the sliding surface control can achieve favorable results when applied to the method of the present invention.
This method can be carried out by a system which comprises means for controlling a fore-and-aft force which is individually applied to each of right and left vehicle wheels; means for detecting a dynamic state quantity of the vehicle including at least a traveling speed; means for determining a cornering force of each of the wheels; and means for computing a yawing moment which a vehicle is desired to produce according to values obtained by said dynamic state quantity computing means and said cornering force determining means; wherein said fore-and-aft force control means is controlled according to a value obtained by said yawing moment computing means.